Light sources having nearly uniform azimuthal luminous intensity have various applications, one of which is navigational signal lighting. Although filament lamps have been heavily relied on in the past for navigational signal lighting, modern light sources, more particularly arc discharge sources, will undoubtedly be employed in increasing members in the future because of the many advantages offered by these light sources. An arc discharge lamp generally provides better efficacy and longer life than its incandescent counterpart. The electrodes are heavier than the filament, so that the lamp may be more rugged. In an arc discharge lamp, the length and width of the arc are design variables to a large extent. In an incandescent lamp, the length and width of the filament are for the most part determined by the lamp wattage. Thus, there is greater flexibility in the choice of optical characteristics of the light source with arc discharge lamps than with comparable incandescent lamps. This is a significant factor in signal lighting particularly with lamps of three hundred watts or less.
The principal object of a signal light is to emit as much light flux as possible from a reliable light source and direct the light into the plane of the horizon. The light may be radiated in all horizontal directions simultaneously, or it may be collected into one or more narrow beams which are mechanically rotated. There are basically two types of rotating beams or beacons. In the first type, a reflector or other means of concentrating the light is used with the lamp. The entire optical system is rotated. This method generally produces a single beam; all of the emitted light is swept through 360 degrees. In the second type, a rotating screen surrounds a stationary lamp. The screen contains multiple lenses or other means for concentrating light. This method generally produces multiple rotating beams, one beam associated with each lens or sector subtended by a lens. The emitted light within any sector is formed into a pencil beam and swept only within that sector.
The observable range of a signal light is directly related to the luminous intensity emitted in the direction of the observer. Where the signal emanates in all directions simultaneously, it is highly desirable for the luminous intensity to be uniform so that the effective range of the signal will be independent of the position of the observer. In the case of a single rotating beam, uniformity of luminous intensity is not as critical because of the integrating effect of the reflector. In the case of multiple beams, uniformity of luminous intensity again is critical, because the integrating effect of a lens is limited to the sector subtended by the lens.
It would be an advancement of the art if an arc discharge lamp could be provided which is well suited for navigational signal applications and in particular, has improved uniformity of azimuthal luminous intensity.